1. Field of the Invention
This invention relates to the manufacture of glass preforms, from which optical fibre waveguides can be produced by drawings using a vapour deposition process. Commonly, the method used is of the type (hereinafter referred to as "the type specified") in which a chemical reaction is caused to take place in a gaseous reaction mixture so as to form a coating on the inner surface of a glass tube wherein the reaction is promoted by the production of a plasma in the mixture.
2. Description of Related Art
Normally the gaseous reaction mixture consists of a carrier gas (usually oxygen) and volatile precursors (usually halides such as silicon tetrachloride). These may be introduced into the region of the tube in which the plasma is produced by way of a perforated central dispenser tube.
This particular method, and forms of the apparatus by which it may be carried out, are disclosed in U.K. Patent Application Nos. 2093829, 2134099 and 2151609, to which reference should be made for further details.
It is advantageous to dope preforms for fibre lasers and fibre amplifiers with rare earth elements. However, the rare earth halides are solids at room temperature and have low vapour pressures relative to silicon tetrachloride even when heated to high temperatures. For example, neodymium chloride has a vapour pressure of approximately 0.65 torr at 1000.degree. C.